Cavity-backed slot antenna



April 10, 1956 J. ASHWELL ET AL CAVITYBACKED SLOT ANTENNA Filed Oct. 11

INVINI'ORS CAVETY-BACIIEB SL8 1 AI AENNA Application Gcto'oer 11, 1951, Serial N 259,874

This application relates to an improved construction for a cavity-backed slot antenna and to a method for making the same. Such antennas broadly are well known to those skilled in the art. However, prior antennas of this type have been subject to certain objectionable features. With prior constructions it has been extremely diflicult to maintain the slot width constant, as is necessary for effective use of the antenna at its design frequency. Moreover, moisture has adversely affected the tuning and impedance characteristics of such antennas. As a result, relatively complex feeding arrangements have been required with means for adjusting the elements for compensating for impedance change produced by the above factors.

It is an object of this invention to provide such an antenna which is inexpensive to build, is relatively unaffected by the presence of moisture, requires but an extremely simple, mechanically rigid feedin system, and which will retain its desired characteristicsdespite rough usage.

A further object is to provide a double-ended T-slot antenna wherein the slot is formed by a block or slab of dielectric material which may be readily and accurately machined to the desired thickness and which will maintain the slot width substantially constant throughout the life of the antenna.

A further object is to provide a simple but highly effective feeding arrangement for such an antenna.

Still another object is the provision of a method for building a cavity-backed slot antenna which will provide a result having a high degree or" permanent ac'curac Further and other objects will appear from the following description and claims when considered in conjunction with the accompanying drawing wherein,

Figure l is a front view of the antenna with parts broken away to better show the antenna.

Figure 2 is a longitudinal sectional view thereof taken on the line 22 of Figure 1.

Figure 3 is a transverse sectional view taken on the line 33 of Figure l.

The antenna comprises a main box-like metal housing 1 having continuous side walls 2 and 3, end walls 4 and 5 and a back wall 6. The end and side walls terminate in a continuous flange 7 surrounding the open front of the housing. This housing can readily be formed in a suitable drawing operation and its dimensions can be held to relatively close tolerances due to its shape and unitary construction. Extending mediady of the interior of the housing is an elongated spacer block or slab 8 of dielectric material which has first been accurately sized so that its thickness represents the desired slot width. Interposed between the spacer block 8 and the side walls 2 and 3 the housing is a pair of metal channel members 9, having their innermost legs 10 extending toward the back of the antenna and in faceto-face contact with the sides or" the dielectric block 8 to form a radiating slot lit therebetweeu. These ele ments are first clamped together by means of threaded rods 11 of dielectric material extending through suitable holes in the flanges 10 and block 8, and having nuts 12 threaded on the opposite ends thereof to form an assembly as best shown in Figure 3.

The web portions 13 of the channels 9 extend laterally outwardly from the block 8 to the side walls 2 and 3, forming, in effect, the front wall of the housing 1, and terminate in a second pair of rearwardly extending legs 15 which, when the assembly 9, 8, 9 has been inserted in the housing, lie in facc-to-face contact with the inner faces of the corresponding walls. Each leg 15 is then rigidly secured to its corresponding wall by means of suitable fasteners 14. These channels 9 are shorter in length than the interior of the housing and are located centrally thereof. As a result, the space between the ends of the channel members and the end walls of the housing forms a pair of transverse openings or slots 15' at the ends of the first mentioned slot so as to produce an over-all, double T-ended, slot antenna backed by the cavity formed below the channels 9.

To excite the antenna, a metal rod 16 extends transversely across the interior of the housing behind the slot and is rigidly secured to the wall 2 thereof as by a suitable bolt 17. Rod 16 terminates just short of the opposite wall 3 and is there electrically connected to the center conductor 18 of a standard co-axial cable connector 19 secured to the outer surface of wall 3 adjacent the back wall of the housing and adapted to receive the end of a standard co-axial feed line (not shown). The outer portion 20 of the connector 19 is of course insulated from the inner conductor 18 by suitable dielectric material 21 and is electrically grounded to the housing in conventional fashion. As indicated in Figure 3, the inner conductor 18 of the co-axial cable connector may be inserted in a suitable aperture 22 in the end of the rod 16 and soldered thereto.

In order to properly match the co-axial feed line to the antenna, the central portion of the back wall 6 of the housing 1 is dished inwardly as indicated at 23. The spacing between the rod 16 and the back wall of the housing is quite critical and must be maintained within close tolerance if a proper impedance match is to obtain. By providing the dished portion of this back wall, not only is the rigidity of the antenna increased, but the spacing between the back wall and the rod 16 may readily be brought to the desired value while still providing sulhcient room on the far wall 3 for theattachment of the co-axial connector 19. Once the required spacing between the rod 16 and the dished portion 23 has been determined for a particular antenna, the dished portion can readily be formed to the required depth in the same operation by which the housing itself is formed, thus greatly facilitating the manufacture of the antenna and insuring uniformity.

As shown in the drawing, dielectric spacer 8 extends in all directions beyond the legs 10 of the channels 9 as indicated at 24. Thus, any moisture which gets into the interior of the housing is kept away from the elec tromagnetic field between the channels 9 and, as a result, will have negligible effect on the tuning of the antenna, while arcing between the channels 9 will not occur even under high power conditions.

As is clearly shown in the drawing, channels 9 and block 8 are located somewhat rearwardly from the open front of the housing. A suitable cover plate 25 of dielectric material is secured as indicated at 26 to the flanges 7 of the housing to make the assembly substantially air and Water tight. However, due to the spacing between this cover plate 25 and the channels 9 and block 8, any moisture which does accumulate on the in ner surface of the cover plate 25 will be sufiiciently far away from the electromagnetic field existing between the 3 channels 9 as to have substantially no efiect on the tuning of the antenna.

The operation of the antenna is believed obvious to those skilled in the art. The rod 16 serves to excite the antenna so that a strong electric field exists across the slot 10' between the adjacent surfaces of the channels 9. The relatively large openings 15' at the ends of the antenna, as well as the relative high capacity between flanges 10, serve to load the slot 10 so as to permit the antenna assembly to resonate at the desired frequency While still permitting its over-all length to be maintained relatively short.

Due to the construction shown, the various parts can be readily manufactured with required tolerances and yet, when assembled as described and shown, will produce an extremely rigid unit which will retain its desired radiation characteristics and tuning and will be substantially unaffected by the presence of any normal amounts of moisture that might be expected.

While other means could obviously be used to excite the antenna, the feeding arrangement shown is especially well adapted for this purpose, requiring but a few readily available parts and being extremely rigid.

Obviously many other changes or substitutions could be made in the antenna shown without departing from the spirit and scope of the invention as defined by the appended claims.

We claim:

1. An antenna comprising a relatively shallow elongated metallic housing having continuous back, side and end walls and having its front side open, an elongated rigid spacer of dielectric material of predetermined constant thickness positioned within said housing and having the surfaces between which the constant thickness is measured substantially parallel to and spaced from said side walls, a pair of elongated metallic members of a length less than that of said housing and positioned on opposite sides of said spacer, said members having inner edge surfaces defining a slot, which edge surfaces are less in both length and width than the said spacer surfaces, said edge surfaces being parallel to and abutting the spacer surfaces and rigidly secured thereto, the outer edges of said members being rigidly secured to said side walls in electrical contact therewith throughout their length, said members and said housing walls co-acting to form a cavity-backed, T-ended slot antenna, the peripheral edge portions of said spacer extending beyond the slot-defining edge surfaces of said members in all directions whereby to minimize detuning eifects of moisture on said antenna.

2. An antenna comprising a relatively shallow elongated metallic housing having continuous back, side and end walls and having its front side open, an elongated rigid spacer of dielectric material of predetermined constant thickness positioned within said housing and having its major surfaces substantially parallel to and spaced from said side walls, a pair of elongated metallic members of a length less than that of said housing and positioned on opposite sides of said spacer with their inner edges parallel to and abutting the major surfaces thereof and rigidly secured thereto, the outer edges of said members being rigidly secured to said side walls in electrical contact therewith throughout their length, said members and said housing walls co-acting to form a cavity-backed, T-ended slot antenna, and means for exciting said antenna comprising a rod extending transversely of said housing behind said members and having one end thereof rigidly secured directly to one of the side walls in electrical contact therewith at a point spaced forwardly from said back wall, the other end of the rod terminating close to but spaced from the opposite side Wall and a conductor extending through and insulated from said last mentioned side wall for rigidly supporting said other end of said rod and for conducting electrical energy thereto, the portion of said back wall adjacent said rod being dished inwardly toward said rod whereby to match the impedance of said excitation means to that of said antenna;

3. A cavity-backed, slot antenna comprising a metallic housing having continuous back, side and end walls and having its front side open, a pair of metallic members spaced forwardly from said back wall and rigidly secured to said side walls in electrical contact therewith, said members extending laterally inwardly from said side walls into relatively closely spaced relationship to one another and having rearwardly extending flanges formed along their inner edges, said flanges being spaced from one another to form an elongated slot of predetermined width therebetween, a rigid spacer of dielectric material having a thickness equal to said predetermined width and lying between said flanges, said flanges and spacer having aligned openings therein, a tie-bar of dielectric material passing through said openings, and abutments on said tie-bar, one lying in contact with each of said flanges for maintaining said flanges in contact with said spacer.

References Cited in the file of this patent UNITED STATES PATENTS 2,402,663 Ohl June 25, 1946 2,431,124 Kees et al. Nov. 18, 1947 2,433,368 Johnson et al. Dec. 30, 1947 2,487,622 Wehner Nov. 8, 1949 2,508,085 Alford May 16, 1950 2,514,678 Southworth July 11, 1950 2,573,460 Lindenblad Oct, 30, 1951 2,658,143 Fiet et al. Nov. 3, 1953 FOREIGN PATENTS 639,270 Great Britain June 28, 1950 OTHER REFERENCES Lindenblad: Slot Antennas, reprint of Proc. 1. R. B, vol. 35, No. 12, December 1947. 

